Electroacoustic transducer



April 1958 R. w. SAMSEL 9 ELECTROACOUSTIC TRANSDUCER Filed Sepg. 1d,1948 Richard W. Samsel,

His Attorneg.

2,832,058 ELECTROACOUSTIC TRANSDUCER Richard W, Samsel, Schenectady, N.Y., assignor to General Electric Company, a corporation of New Yorlr Myinvention relates to electroacoustic transducers and more particularlyto underwater sound transducers utilizing piezoelectric crystals whereina lens of sound transmitting material is disposed between the crystalelement or elements and the water medium into which the sound isdirected or from which the sound is received.

An object of my invention is to provide an improved wide beam angleelectroacoustic transducer.

A more specific object is to provide an improved unitary transducerincorporating a sound lens to determine the beam width through whichsound is to be directed or is to be received.

A further specific object is to provide a wide beam angle piezoelectriccrystal transducer comprising a fiat supporting surface for the crystalelements.

An additional object is to minimize damping effects on the crystalelements of a piezoelectric crystal transducer.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claim. My inventionitself, however, both as to its organization and method of operation,together vw'th further objects and advantages thereof may best beunderstood by reference to the following description taken in connectionwith the'accompanying drawing in which Fig. 1 is a back view, partiallycut away, and Fig. 2 is a cross sectional side view of a transduceraccording to my invention; Figs. 3 and 4 are enlarged detail sectionalviews ofportions of the transducer; and Fig. 5 is a chart showingtypical operational characteristics of a transducer according to myinvention.

Referring to Figs. 1 and 2 of the drawings, a plurality of piezoelectriccrystal elements 1 are shown arranged in a regular pattern across theback surface of a flat circular disc 2 of sound-transmitting materialsuch-as .a methyl methacrylate plastic. The crystals may be L-cutRochelle salt crystals each having a separate conductive foil electrodeof tin or other soft metal substantially coextensive with each majorface of each crystal element. Foil electrodes 3 covering the exposed orback major face of each crystal are visible in Fig. 1, and the positionof electrodes 3 as well as of electrodes 4 covering the front major faceof each crystal is shown in Figs. 2, 3 and 4. The foil electrodes arepreferably applied to both major faces of each crystal by first coveringthe face with a thin film or coating of liquid cement and then layingthe foil across the face and rubbing the exposed surface of the foilwith a smooth, rounded object.

The crystal elements with electrodes in place are cemented to the backsurface of disc 2, preferably with a rubber cement with ethylenedichloride solvent. The

end portion of a connecting foil tab 5 is interposed at one,

edge of each crystal element to provide an electrical connection to oneelectrode of each. Tabs 5 are shown in Figs. 1 and 2, but in greaterdetail in Fig. 3, wherein tab 5 is arranged with end portion 6underlying crystal element 1 and disposed between electrode 4 and disc2.

2,832,058 Patented Apr. 22, 1958 While electrodes 3 and 4 and the endportion 6 of tab 5 are shown in Fig. 3 as having substantial thickness,in an actual device the foil is very thin, ofthe order of onethousandthof an inch or less, and the surfaces of the crystal and disc are veryclose together. Since the crystal electrode 4 is cemented to the crystalface, and cemented additionally to the flat back face of disc.2, thecrystal is held firmly against the disc and the thin layers or films ofcement and the foil do not substantially affect the transmission of themechanical vibration of the crystal into the disc 2. As shown in Fig. 3,tab 5 extends from the area of contact with electrode 4 between adjacentcrystal elements 1 to a free end. Slips 7 of an insulating material,such as a synthetic resin, are arranged on either side of tab 5 toinsure against accidental contact with electrode 3 and to give supportto tab 5.

The free ends of tabs 5, as shown in Figs. 1 and'2, are

wrapped about and preferably soldered with a low temperature metalsolder to bus members 8 which are in turn connected to and supported bya peripheral bus mem-" ber 9.

To provide connections to the exposed electrodes 3, as shown in Figs. 1,2 and 4, narrow strips of foil10 are cemented to the electrodes 3 on theback faces of rows of the crystals 1 and are connected by wrapping aboutand soldering to a peripheral bus member 11, thus serving to connect allthe back electrodes 3 of all of the crystals together and to the busmember 11. Peripheral members 9 and 11 are supported by insulatingblocks 12 of synthetic resin or plastic and are disposed in holesdrilled through the blocks. In order that soldering and consequentheating of copper bus members 9 and 11 will not melt the blocks 12,small heat resistant sleeves 13 of fiber or other'material line each ofthe drilled holes in the blocks. Bus members 9 and 11 of the two bussystems terminate in a suitable dual contact receptacle 14 forconnecting all of electrodes 3 to one terminal of an external apparatusand all of electrodes 4 to another terminal. The external apparatus maybe, for example,

an underwater sound transmitter between the output ter-- minals of whichappears an exciting voltage for the crystal transducer, or it may be areceiver with input terminals arranged to receive the voltages producedacross the pairs of electrodes of each crystal in response to receivedsound waves. a

The complete transducer unit comprises the disc 2, a hollow cylindricalmember 15, a back cover plate 16 and a sound lens 17. These parts may beassembled by various methods, but the assembly is probably most con-2veniently accomplished by casting the lens 17, which is preferably ofplastic material, and placing the disc 2 against the flat face thereofprior to hardening of the lens whereby these two members will be bondedtogether. The cylindrical housing member 15 may be cemented peripherallyaround disc 2, and to the peripheral back flat face portion of lens 17.The back cover plate 16 is cemented and bolted in place with a gasket 18and bolts 19 cooperating with the housing. An inwardly extending flange20 assists in maintaining a tight joint, preventing any leakage adjacentbolts 19.

Housing member 15, and cover plate 16, may both be of methylmethacrylate. Flange 20, shown as part of housing member 15, may be aseparate ring member cemented to' the housing. Gasket 18 is preferablyof rubber.

Many of the parts mentioned are identified in each of several of thefigures to assist in an understanding of the invention. As shown in Fig.1, connecting receptacle 14 is so mounted as to provide an air and watertight seal with the housing member 15, and the crystal elements are thusenclosed in a sealed chamber. The atmosphere in p asaaoes the chambermay be air or any desired gas. For convenience in mounting the completetransducer assembly,

invention has anindex of refraction for sound less than the index forthe medium in which it is intended for use. The exposed front face ofthe lens is preferably spherical.

The transmitter operating, alternating current electrical energy of, forinstance, several hundred kilocycles is provided between bus members 9and 11 from suitable external apparatus through the connector 14 toexcite the vibratile elements 1. The vibration of these elements istransmitted through disc 2 and lens 1'7 to the external medium, whichmay be water, and leaves the outer curved face of the lens as a broadbeam centered about the lens axis. Since the back face of each elementis in contact only with air, there is very little damping of theelements, and the transducer has a high efficiency. The beam .is broadwith the spherical lens as shown and described because the speed ofsound or compressional waves through the material comprising the lens isgreater than the speed in water. Fig. is a polar plot diagram showingthe typical relative intensities of the transmitted beam in differentdirections from the face of the trans-J ducer, the beam pattern beingcentered about the lens axis. The transducer unit is identified in thisfigure by reference numeral 22. sensitivity of the transducer is also inaccord with this diagram. It will be understood that this diagram isexemplary and approximate only'since the shape of the beam differs fordifferent frequencies and in accord with difierences in otherconditions. I

When used for reception, the elements 1 may be excited to, generateelectrical energy by compressional waves approaching the lens fromdirections within awide angle as measured from the lens axis. The energythus generated is collected from the crystal element electrodes andappears as a signal between busses 9 and 11.

The transducer is particularly adapted to underwater object locating andmay serve as atransmitter, as a receiver, or as transmitter andreceiver. It may be desired, for instance, to energize the elements totransmit compressional waves at a frequency of, for instance, severalhundred kilocycles for an interval of time, to then disconnect the bussystem 9, 11 from the external signal generator and connect the bussesto cathode ray display apparatus arranged to indicate the reception ofechoes of the previously transmitted frequency received afterreflectionthereof from submerged objects. In such applications thetransducer may be mounted below the water When used for reception, the.

k line external to the hull of a vessel, within a suitable housing ifdesired, and arranged with mounting means for orienting the transducerin a desired direction and, conveniently, including means for changingthe orientation. The signal generating and display apparatus may bearranged within the hull with connections to the busses 9, 11 extendingthrough the hull.

While I have shown onlycertain preferred embodiments of my invention byway of illustration,-modifications will occur to those skilled in theart and I therefore wish tohave it understood that I intend, in theappended claim, to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of theUnitedStates is:

An electroacoustic transducer comprising a circular solid plastic lensmember with a convex front face and planar back face, a solid plasticdisc member of substantially the same diameter assaid lens firmlyaffixed by one .face to said back face, a plurality of piezoelectriccrystal elements each with two electrodes covering opposite major facesthereof, each of said elements being closely spaced and firmly afiixedto the other face of said disc at an electrode covered major face ofeach element/said plurality of elements being substantially coextensivewith said disk, two'elect'rical conductive bus systems interconnectingrespective electrodes of said elements in a polarity such that in-phasevibration of said crystal elements corresponds to a voltage between saidbus systems of a phase of predetermined relation to said vibration, oneof said bus systems having insulated conductors passing betweenadjacently mounted ones of said elements, each conductor.makingconnection to the electrode covering the afiixed face of adifferent element, and means forming a sealed casing for said elements,said last means comprising at least one of said members, and aconductive portion of each of. said bus systems sealed through said lastmeans to provide external connections to said electrodes.

References'Cited in the file of this patent UNITED STATES PATENTS2,283,285 Pohlman May 19, 1942 2,323,030 (.vruetz mac'herv June 29, 19432,399,820 Morgan May 7, 1946 2,405,186 Benioff Aug. 6, 1946 2,414,827Mason Ian. 28, 1947 2,438,936 Mason ..l..;. Apr. 6, 1948 2,447,333 HayesAug. 17, 1948 2,448,365 Gillespie "Aug. 31, 1948 2,452,068 Peterson Oct.26, 1948 2,477,246

Gillespie July 26, 1949

